This invention relates to a code division multiple access (CDMA) system and, in particular, to an automatic gain control (AGC) loop for a CDMA system.
CDMA systems intentionally combine a large number of different transmissions on the same channel at the same time. CDMA allows the different transmissions to interfere with each other in a controlled fashion. Although signals from different transmissions are combined on the same channel, the information of each transmission is recoverable because each transmission is subjected to a special additional level of encoding before it is sent. Each transmitter has its own unique code and it imparts an extra level of its unique coding on each signal transmission being transmitted. Therefore, although many different transmissions are combined at the receiving end, each transmission maintains the unique signature (coding) imparted to that transmission. The receiver, in turn, "knows" the unique code of the signal it is looking for, and by applying this code as a kind of filter to the stream of radio energy in the channel, it can recover the desired signal from the background of all the other signals.
To keep the power levels constant and the dynamic range of the CDMA system as high as possible the analog portion of the CDMA system processing the received analog CDMA signals requires the use of an automatic gain control (AGC) circuit.
Due to the nature of wireless transmission over the air and the activity of different users transmitting on the same channel the received signal power will have a very large dynamic range, for example 50 dB. In order to have circuitry which can reasonably process received signals which have such a large dynamic range, the received signal must go through some sort of automatic gain control (AGC) circuitry to narrow down the fluctuation before the received signal is applied to circuitry which is used to process the signal and to extract information from the signal. It should be noted that the power of the received signal does not carry information. This explains why information is not lost when the power is adjusted using an AGC circuit.
To better utilize the hardware processing the received signal, it is desirable that the AGC circuit maintain the power of the received signal as constant as possible. This is particularly so at the interface between the AGC circuit and a subsequent analog-to-digital converter (ADC). In order to maintain the power constant with a high degree of accuracy there are two sources of power fluctuation which have to be dealt with. One is the very rapid fluctuation resulting from the power dynamic range in air due to multi-path fading, constant movement of the transmitter (mobile) relative to the receiver (cell-site) and similar conditions. The second is a very slow fluctuation due to temperature changes, component aging, cable loss, and other component inaccuracies. It would be very expensive and difficult to create a single AGC circuit to deal with these two sources of fluctuation. Instead, as shown in FIG. 1A, it is more feasible to use two stages of AGC circuitry. The first stage of AGC is used to perform, fast and gross adjustments which bring the power level down to a narrow range. The second AGC stage is used to perform slow and fine adjustments to bring the power level to a level which is equal to an ideal level.
A second AGC stage for processing CDMA signals in accordance with the prior art is of the type shown in FIG. 1B. FIG. 1B illustrates a system in which a power sensing module is used to sense and control the gain of a baseband amplifier. In response to an analog input signal, a demodulator 6 produces an in-phase (I) and a quadrature (Q) signal which are respectively applied to baseband amplifiers (8a,8b). The power level of the signal at the outputs of amplifiers 8a and 8b is sensed by means of power sensing (9a, 9b) modules which are used to control the gain of the baseband amplifiers. To implement the second stage of the AGC circuit requires a module to sense the power of the received signal very accurately and to make a slow adjustment to the baseband amplifier. The power sensing module (9a, 9b) is required to perform an accurate power metering function and is generally a very expensive and complex analog circuit. The module will constantly monitor the input power and will decrease or increase the gain of the baseband amplifier as a function of whether, or not, the power measured over a last preceding fixed interval exceeds a target point.
However, the circuit of FIG. 1B and similar circuits suffer from some problems. It is very difficult and expensive to implement analog circuitry to sense power accurately over a wide temperature range. Analog components tend to be inaccurate and are subject to aging factors. In a CDMA system, the goal of the system is to process the received signal and generate a fixed statistical histogram of the sampled input signal. In accordance with the prior art scheme, discussed above, the system is designed to maintain a fixed power level. This is an indirect way of trying to obtain an ideal statistical histogram. Moreover, maintaining the power level within a fixed range does not ensure that the sampled digital data will yield an ideal histogram due to the inaccuracy in the AGC conversion circuit.